1. Field of the Invention
The present invention relates to a phase-change optical recording medium, a method of recording data on to the phase-change optical recording medium, and an optical recording apparatus which records data on to the phase-change optical recording medium.
2. Description of the Related Art
As a rewritable optical disk (rewritable optical recording medium), a “phase-change optical disk” as typified by a CD-RW or a DVD-RW is known. The phase-change optical disk has a phase-change recording film whose status changes between the crystalline status and the amorphous status in a reversible fashion. Data is recorded on to the phase-change optical disk by irradiating the recording film with a laser beam and thereby changing a reflection coefficient of the recording film or an optical phase. The recording film is in the amorphous status immediately after the optical disk is manufactured. In the initializing process, the optical disk is irradiated with a laser beam, and thus the recording film is crystallized. After that, the recording film at a desired spot is amorphized by irradiating the desired spot on the optical disk with a laser beam, and thereby the data is recorded. The amorphized spot is called a “mark (amorphous mark)”.
Known as compositions of the recording film of a typical phase-change optical disk are GeTe, SbTe, GeSbTe, InSbTe, AgInSbTe and so on.
For example, Japanese Laid Open Patent Application JP-P2002-240432A discloses an optical recording medium having at least a phase-change optical recording layer. The phase-change optical recording layer is constituted mainly of Sb(x)-Te(y) wherein x/y=1 to 4. In addition, the phase-change optical recording layer includes at least one element selected from the group of B, Al, Si, Ga, Ge, Ag, In, Sn, Ba, La, Au, Bi and Gd as an additional element. For example, the phase-change optical recording layer is constituted of GeGaSbTe, in which relative proportions of respective elements are Ge-3 at %, Ga-7 at %, Sb-65 at %, and Te-25 at %.
Japanese Laid Open Patent Application JP-P-Heisei-9-293269 discloses another phase-change optical recording medium. The phase-change optical recording medium has a substrate, a first dielectric film on the substrate, a recording film on the first dielectric film, a second dielectric film on the recording film, and a reflective film on the second dielectric film. The recording film is constituted of (GeSbTe)x(InySb1-y)1-x. Thus, crystalline “GeSbTe” and amorphous “InSb” coexist in the recording film.
Recently, a variety of signal processing methods are introduced for the purpose of increasing the storage capacity of the phase-change optical disk. Also, a “land and groove recording method” is introduced, in which data are recorded on to both land and groove used for tracking. Also, a super-resolution playing technology is introduced, which enables the playing of fine marks smaller than an optical diffraction limit.
Also, there are increasing needs for an increase in recording speed as well as an increase in storage capacity. As for optical disk drives supporting the CD-R and the DVD-R, for example, such disk drives with improved recording speed are continuously marketed.
In order to realize an increase of the recording speed (referred to as a “high-speed recording” hereinafter) in the phase-change optical disk, it is necessary to increase a crystallization speed of the phase-change recording film. It is desired to improve the crystallization speed and hence ensure a sufficient erasability even in a condition of a high linear velocity.
For example, Japanese Laid Open Patent Application JP-P2001-322357A discloses a technique for improving the crystallization speed of the recording film. According to the technique, in a recording film of GeTe-Sb2Te3 series, a part of GeTe is replaced by SnTe. This idea utilizes a nature that the crystallization speed of SnTe is higher than that of GeTe. The increase in the crystallization speed is realized by replacing a portion of GeTe by SnTe.
However, generally speaking, when the crystallization speed is increased, crystallization temperature of the recording film is lowered. As a result, the recorded data (amorphous mark) is more likely to disappear. In other words, the increase in the crystallization speed causes deterioration of data holding characteristic (storage stability). Moreover, the increase in the crystallization speed causes increase in a so-called “cross erase”. The cross erase means a phenomenon that when data is recorded on a certain track under a condition of narrow track pitch, a mark existing on the adjacent track is erased. Although the cross erase is more likely to occur due to the narrow track pitch as mentioned above, narrowing the track pitch is important from a view point of increasing the storage capacity.
It is desired not only to improve the crystallization speed but also to reduce the cross erase. That is to say, it is strongly desired to establish a technique which can realize both the “high-speed recording” and the “high storage capacity” in the phase-change optical disk.